I would highly suggest first backing up any information you wish to save, as Bob Smith said, it WILL wipe your drive clean. I also suggest using the second method that he suggested.
By inserting your Windows CD and rebooting, you can save yourself some steps. On rebooting, you will be presented with the option to boot from CD, do so. Follow the onscreen instructions. Make sure that you have a serial number ready to use to reinstall windows. If you've lost your key, you can recover it from your current installation using a tool called "RockXP." Google it.
When formatting, I suggest going with the NTFS partition (Windows XP or later) when presented with the option. Do not use the fast format option, which will not scan for errors on the hard disk, possibly causing you problems down the road.
As Bob Smith put it, Good luck!
Ryeguy, January 2009